Mapping a FabricPath LID to an Outbound Interface

When a FabricPath edge switch needs to send a frame to a remote MAC address, it performs a MAC address table lookup and finds an entry of the form SWID.SSID.LID. The SWID represents the switch-ID of the remote FabricPath edge switch, the SSID represents the sub-switch ID (which is only used in vPC+) and the LID represents the outbound port on the remote edge switch. However, the method by which these LIDs are derived doesn’t seem to be very well documented and this had been bugging me for a while. So I decided to dig in and see if I could find out a bit more about the way LIDs are used on the Nexus switches. Continue reading

Decoding DCBX

As I was studying for the Troubleshooting Cisco Data Center Unified Fabric (DCUFT) exam, I came across a couple of low level NX-OS commands that can help determine whether the Data Center Bridging eXchange (DCBX) protocol is functioning correctly. Being able to verify the operation of DCBX is important when troubleshooting FCoE, because the proper operation of the Data Center Bridging (DCB) extensions is a prerequisite for FCoE.

Unfortunately, the output of these commands is rather cryptic, because it essentially shows the content of the DCBX TLVs as raw hex dumps, rather than nicely decoding the fields in the output of the command. Because I still wanted to understand how to read the DCBX information contained in these commands, I decided to dive a bit deeper into the DCBX protocol. Continue reading